Thermoplastic polyolefin substrates are widely used in parts for automotive vehicles, container caps and containers for cosmetics, household appliances as well as in other applications. Often, organic coating compositions are applied to such polyolefin substrates for decorative as well as protective purposes. However, an ongoing problem with respect to such use of these substrates has been the difficulty in achieving adequate adhesion of organic films to the thermoplastic polyolefin. The problems are particularly difficult in automotive vehicle applications where, in addition to the requirement that decorative and protective coatings adhere well to the polyolefin substrate, there are other requirements which are very difficult to achieve in respect to utilization of thermoplastic polyolefin substrates, such as requirements for a high degree of resistance of the finally coated part to organic solvents (e.g., gasoline) and a high degree of resistance of any hardened coating to humidity.
A number of proposals have been made for overcoming the adhesion problem ranging from subjecting the polyolefin substrate to corona discharge to the application of various primer compositions to the substrate for improving adhesion of organic coatings thereto. However, while such proposed techniques have achieved a fair measure of success with respect to the adhesion problem, there remain various disadvantages such that the goal, for example, of achieving a desired combination of excellent adhesion, solvent resistance and humidity resistance in an advantageously economic manner has remained elusive. For example, one such proposal described in U.S. Pat. No. 4,683,264 involves use of a hardenable coating composition containing a polyolefin resin which has been first reacted with an unsaturated polycarboxylic acid or anhydride to produce an intermediate, modified polyolefin product having a saponification value (acid value) in the range of from 6 to 60 which intermediate product is subsequently chlorinated to a specified degree. Thereafter, the chlorinated product is formulated with a compound or resin having at least two epoxy groups per molecule in a specified ratio.